The invention relates to electric lamps and particularly to tungsten halogen electric lamps. More particularly the invention is concerned with tungsten halogen lamps with safe electrical disconnects.
Ordinary incandescent lamps are commonly made with a tungsten filament in an inert atmosphere. If the outer envelope is accidentally broken, oxygen rushes into the low pressure cavity. The hot tungsten filament then oxidizes and fails. This happens very rapidly, perhaps in a tenth of a second. To increase the efficiency and life of tungsten filament lamps one may use a halogen fill. Tungsten halogen lamps, as they are then called, are operated at significantly higher temperatures. The lamp capsule can then cause injury if not properly contained. To cope with hazards, tungsten halogen lamps have been made with outer jackets to create a greater heat dissipating surface and a containment vessel. One effective design uses a thick heavy (one eighth inch) walled outer envelope. The thickness of this outer wall is five or more times greater than that of a common incandescent lamp. The material cost of the thicker wall is apparent. The thicker wall also absorbs more of the light passing through it, and the weight can be objectionable in some lamp fixtures. There is a need to provide a thin outer walled tungsten halogen lamp.
The problem with a thin outer walled lamp system is that the outer envelope may break, while the inner envelope remains intact. The inner capsule then continues to operate, generating its normal heat. The broken outer capsule situation is then seen by some to pose a possible burn or fire hazard. A known way to extinguish the inner lamp is to use an electrical interrupt system responsive to the failure of the outer envelope. Such electrical interrupt systems have used mechanical or pressure based switches to cut the power to the inner capsule, on the fracture of the outer envelope. These mechanical systems are generally seen to be too expensive to be economically viable in the low wattage lamp market. An alternative method uses a fuse like system, so that when the outer envelope breaks, a fuse burns and thereby electrically disconnects the inner lamp capsule. Internal fused lamps have been used in high wattage discharge lamps where the fuse is sufficiently hot to ignite on contact with air. Low wattage lamps have been found to not generate sufficient heat to ignite the fuse. Additionally, actual construction of fused lamps can be impractical on large scale as welding the fuse in place can ignite the fuse. To avoid this one could use an inert assembly atmosphere, but that would be costly and would slow assembly. There is then a need for a lamp structure and a method of assembling it that enables a safe, thin outer walled tungsten halogen lamp.
U.S. Pat. No. 4,192,5,251 shows a high intensity arc discharge lamp that has an arc tube the emission of which includes some harmful UV radiation. An outer envelope surrounds the arc tube and substantially blocks the harmful UV radiation. The lamp includes a means to render the arc tube inoperative when the outer envelope becomes cracked or punctured sufficiently to admit air. The means includes a pyrophoric material to initiate combustion of a current carrying portion of the lamp circuit.
It is an object of the invention is to enable a safe, thin walled outer envelope tungsten halogen lamp.
It is an object of the invention is to provide a lamp with an oxygen sensitive fuse operable in low wattage lamps.
It is an object of the invention is to provide a method of manufacturing a pyrophoric fuse sensitive to moderate temperatures.
It is an object of the invention is to provide an inexpensive and practical, low wattage, thin walled tungsten halogen lamp with a pyrophoric fuse.
It is an object of the invention is to provide an inexpensive and practical, low wattage, thin walled tungsten halogen lamp with a pyrophoric fuse that is extinguished in five seconds or less on fracture of the outer envelope.
An incandescent lamp may be made with a sealed, light transmissive tungsten halogen capsule receiving electrical power through a first lead and a second lead. At least one of the leads is coupled in series with an oxidizable electrical connection having a first pyrophoric stage ignitable at a first temperature and a second pyrophoric stage ignitable at a second temperature higher than the first temperature. The first pyrophoric stage is positioned sufficiently near the second pyrophoric stage to transmit sufficient heat from the first pyrophoric stage during pyrophoric ignition to the second stage to cause ignition of the second stage. A sealed light transmissive outer envelope encloses the capsule, the oxidizable connection and an inert atmosphere around the connection. An electrical connection for the tungsten halogen capsule is also provided.